This invention relates to a recording apparatus in which a sheet of recording paper is intermittently moved or sub-scanned so that recording is made line after line.
In many recording systems such as printers or facsimile equipment using a thermal recording head, the thermal recording of one line is attained in discrete portions by driving the thermal head several times because of the limitations of the capacity of the electric source or the like (hereinafter referred to as a "one-line-partitioning printing system"). In such a recording apparatus, the recording has to be attained on each line while the recording paper is stopped, otherwise the printed dots on the recording paper may be mispositioned or replaced in the sub-scanning or moving direction of the recording, resulting in the deterioration in quality of the recorded picture. For this reason, in the above-mentioned recording apparatus, a pulse motor is used in a mechanism for intermittently moving or sub-scanning the recording paper to move the recording paper intermittently line a after line and the thermal head is driven during stoppage of the recording paper.
A so-called blank line skipping system in which sub-scanning is made at a high speed on a line for which there exists no picture information to be recorded (hereinafter referred to as an "entire white line") is also utilized in the recording system of the type as mentioned above in accordance with the demand for high speed recording. In the recording apparatus in which the blank line skipping system is employed, however, there has been a problem that printed dots may be mispositioned or replaced at the first recording line after a blank line skipping.
FIG. 1 is a schematic diagram for explaining the cause of such displacement of the printed dots. In FIG. 1, the y-axis represents the distance of movement of the recording paper and each of the sections defined by dotted lines perpendicular to the y-axis represents the line interval of the recording paper. The x-axis represents the lapse of recording time and each of the sections defined by dotted lines perpendicular to the x-axis represents the processing speed per line (sum of the time during which the recording paper moves and the time during which the recording paper rests so that the recording operation is effected). In FIG. 1, lines l.sub.1 and l.sub.2 are entire blank lines and lines l.sub.3 to l.sub.5 are recording lines on which a recording has to be made.
In FIG. 1, the processing speed is a high speed one of 2.5 msec/line at each of the entire white lines l.sub.1 and l.sub.2 by employing the blank line skipping system, while a lowered speed of 5.0 msec/line at each of the lines l.sub.3 to l.sub.5 is used because there is black picture information to be recorded. The sub-scanning of the recording paper is effectuated intermittently and the recording is made within a period of time T at the rear part of the sub-scanning period. Ordinarily, the period T is set as a maximum value which is large enough to stop the recording paper at a predetermined sub-scanning speed, responding to the demand of speeding up the recording. At the third line l.sub.3, however, the sub-scanning mechanism cannot sufficiently follow the sudden decrease of the rotary speed of the pulse motor due to the decrease of the sub-scanning speed. Thus, at the line l.sub.3, the stoppage period t becomes shorter than the recording period T so that a part of recording is made during the traveling of the recording paper. This causes the displacement of the printed dots, resulting in deterioration in quality of the recorded picture.